Molecular Dynamics of Ethylene Glycol Dimethacrylate Glass Former: Influence of Different Crystallization Pathways

Abstract

The crystallization induced by different thermal treatments of a low molecular weight glass former, ethylene glycol dimethacrylate (EGDMA), was investigated by dielectric relaxation spectroscopy (DRS) and differential scanning calorimetry (DSC). The fully amorphous material, dielectrically characterized for the first time, exhibits three relaxation processes: the alpha-relaxation related to dynamic glass transition whose relaxation rate obeys a Vogel-Fulcher-Tamman-Hesse (VFTH) law and two secondary processes (beta and gamma) with Arrhenius temperature dependence. Therefore, the evaluation of distinct crystallization pathways driven by different thermal histories was accomplished by monitoring the mobility changes in the multiple dielectric relaxation processes. Besides isothermal cold-crystallization, nonisothermal crystallizations coming from both the melt and the glassy states were induced. While an amorphous fraction, characterized by a glass transition, remains subsequent to crystallization from the melt, no alpha-relaxation is detected after the material undergoes nonisothermal cold-crystallization. In the latter, the secondary relaxations persist with a new process that evolves at low frequencies, designated as alpha' that was also detected at advanced crystallization states under isothermal cold-crystallization. Under the depletion of the alpha-relaxation, the beta-process when detected becomes better resolved keeping the same location prior to crystallization leading to a decoupled temperature dependence relative to the alpha-process.

Real-Time Monitoring of Molecular Dynamics of Ethylene Glycol Dimethacrylate Glass Former

“…Equation 6 explains the linear dependence of ∆h cr with temperature and allows determining ∆c p ) c pl -c ps from the experimental results, obtaining a value of ∆c p ) c pl -c ps ) 138.6 J · (K · mol) -1 , slightly higher than the heat capacity difference between the liquid and the glass (∆c p (T g ) ) 114.9 J · (K · mol) -1 determined in heating scans). 1 The fit of eq 6 to the experimental values yields h l (T m ) -h s (T m ) ) 21.4 kJ · mol -1 , close to the values determined by integration of the melting endotherms detected during the heating scans; however, it can not be unambiguously concluded that complete crystallization from the melt was attained as probed by DSC.…”

“…The difference in the crystallization behavior of EGDMA between the two techniques was also found in a previous work under the same temperature protocols. 1 While the estimated induction time for cold-crystallization, in the studied temperature interval, increases with a decrease in temperature, for melt-crystallization that covers a more extended temperature range, and t 0 generally follows the temperature dependence observed for the experimental time of onset for crystallization shown earlier in the inset of Figure 6b.…”

“…16 For EGDMA, with the progress of crystallization, the R-process depletes with no significant changes either in position or in shape. 1 A similar behavior is reported for the time evolution of the R-relaxation during crystallization in other low molecular weight materials, namely, isooctyloxycyanobiphenyl 15 and pharmaceutical drugs. 17,18 Nevertheless, the invariance in the peak position under crystallization is not universal, e.g., in sorbitol.…”

“…The crystallization enthalpy decreases with undercooling below the melting point, T m -T cr , and is always lower than the melting enthalpy (∆h m ) 20.2 ( 0.6 kJ · mol -1 ). 1 The reason for that is obviously the temperature dependence of the enthalpy of the liquid and the solid according to their respective heat capacity c pl and c ps . Thus, assuming that the difference ∆c p ) c pl -c ps is independent of temperature, the enthalpy increment of the crystallization at temperature T cr can be written T m being the equilibrium melting temperature.…”

“…6,7 The respective dielectric characterization revealed, besides the R-relaxation process associated with the dynamical glass transition, two secondary relaxations, and γ, on decreasing order of temperature. The dynamical behavior of EGDMA (n ) 1) fits in the trend of their counterparts, finding that: (i) the onset of calorimetric glass transition increases with the number of ethylene glycol moieties (T g ) 176, 181, 187, and 190 K, respectively, for n ) 1 to 4), the same happening with the R-process that shifts to higher temperatures with n increasing; (ii) the faster γ-process is independent of the size of the ethylene glycol group; and (iii) the -relaxation slightly shifts to higher relaxation times with the increase of n. 1,7 Due to the ability of EGDMA to crystallize, its real-time isothermal crystallization can be monitored by both differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). Both techniques were used to monitor isothermal cold-crystallization at several temperatures, T cr , nearly above the glass transition temperature (T cr /T g within the range 1.06 to 1.12).…”